Ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX) and meta-xylene (MX) are present together in many C8 aromatic product streams from chemical plants and oil refineries. While all these species have important uses, market demand for para-xylene, used extensively as starting material for making synthetic fibers, is greater than for the other C8 aromatic isomers.
Given the higher demand for PX as compared with its other isomers, there is significant commercial interest in maximizing PX production from any given source of C8 aromatic materials. However, there are two major technical challenges in achieving this goal of maximizing PX yield. First, the four C8 aromatic compounds, particularly the three xylene isomers, are usually present in concentrations dictated by the thermodynamics of production of the C8 aromatic stream in a particular plant or refinery. As a result, the PX production is typically limited to the amount originally present in the C8 aromatic stream, which is, again in the typical case, approximately 24 mol % at thermal equilibrium, unless additional processing steps are used to increase the amount of PX and/or to improve the PX recovery efficiency. Secondly, the C8 aromatics are difficult to separate due to their similar chemical structures and physical properties and identical molecular weights.
A variety of methods are known to increase the concentration of PX in a C8 aromatics stream. These methods normally involve a loop system comprising a separation step, in which at least part of the PX is recovered (and removed from the system in a PX-enriched stream), leaving a PX-depleted stream, the latter being sent to a xylene isomerization step, in which the PX content of the PX-depleted stream is returned back towards thermal equilibrium concentration and recycled to the separation step.
The separation step may be accomplished using fractional crystallization techniques, which are based on the difference on the freezing points of the C8 aromatic isomers, or adsorption separation techniques, which are based on the selectivity of adsorbent for one isomer over another. Amongst the well-known adsorption separation techniques are the UOP Parex™ Process and the Axens Eluxyl™ Process.
A prior art system including the separation step and isomerization steps referred to above generally will include the use of numerous fractionation towers, e.g., a reformate splitter, a benzene recovery tower, a toluene recovery tower, a xylene rerun tower, an isomerization unit heptanizer, and one or more towers associated with the adsorption separation unit, e.g., Eluxyl™ adsorptive separation unit(s). A system comprising a Eluxyl™ adsorptive separation unit using PDEB (para-diethylbenzene) as a desorbent (“heavy” Eluxyl™ adsorptive separation unit) will have an extract tower, raffinate tower(s) and finishing tower(s) while a system comprising a Parex™ adsorptive separation unit using toluene as a desorbent (“light” Parex™ adsorptive separation unit) only needs the extract and raffinate towers, since the extract tower separates out both the toluene in the desorbent stream as well as trace toluene in the xylene feed. In a plant using both types of units the light extract tower can serve as the finishing tower for the heavy unit.
The isomerization step typically is accomplished by contact with a molecular sieve catalyst, such as ZSM-5, under appropriate conditions to convert a para-xylene-depleted mixture of C8 aromatic hydrocarbons to thermodynamic equilibrium amounts. Historically xylene isomerization has been accomplished in the vapor phase, however recently liquid isomerization units have found increasing use in para-xylene separation systems.
It is known that liquid phase isomerization technology can reduce energy usage in an aromatics plant by reducing the amount of feed to vapor phase isomerization. Vapor phase isomerization requires more energy due to the phase change in the isomerization process. In addition, vapor phase isomerization requires more fractionation energy in the isomerization system's heptanizer and xylene rerun tower.
In a simulated moving-bed apparatus, such as a Eluxyl™ unit, examples of adsorbents include charcoal, ion-exchange resins, silica gel, activated carbon, zeolitic material, and the like. An adsorbent, which is particularly useful for separating para-xylene from other C8 aromatics, is a faujasite-type molecular sieve material, such as zeolite X or zeolite Y, optionally, substituted or treated with an enhancing agent, such as a Group I or II element, such as potassium or barium. Examples of adsorbents for separating para-xylene from other C8 aromatics are described in U.S. Pat. No. 3,761,533.
It has recently been discovered that certain Metal Organic Frameworks (MOFs) are para-xylene selective adsorbents, whereas other MOFs are ortho-xylene selective adsorbents. The para-xylene selective MOFs have a greater affinity for para-xylene than other C8 aromatics. In U.S. Pat. No. 8,704,031, two MOFs are said have excellent selectivity to para-xylene, and another MOF is said have good selectivity for ortho-xylene. The para-xylene selective MOFs are A1-MIL-53 and Zn-MOF-5. The ortho-xylene selective MOF is Cr-MIL-101. These MOF adsorbents may be used in a simulated moving bed unit. U.S. Pat. No. 8,704,031 also describes desorbents for such MOFs. Such desobents include aromatics, such as para-diethylbenzene, toluene and 1,4-diisopropylbenzene.
The extract stream from a simulated moving bed unit for separating PX from MX, OX and EB includes para-xylene and desorbent. The raffinate extract stream from such a simulated moving bed unit includes MX, OX, EB and desorbent. Desorbent is separated from C8 aromatics by distillation.
Para-diethylbenzene has a boiling point at atmospheric pressure of 183.9° C. Toluene has a boiling point at atmospheric pressure of 110.6° C. C8 aromatics have boiling points at atmospheric pressure within the range of about 136° C. to about 145° C. Accordingly, in order to separate desorbent from C8 aromatics by distillation the extract or raffinate must be heated above the boiling point of C8 aromatics, when diethylbenzene is used as the desorbent, and the extract or raffinate must be heated above the boiling point of toluene, when toluene is used as the desorbent. These distillations require the use of considerable amounts of energy.
Accordingly, it would be desirable to provide a simulated moving-bed adsorptive separation process for separating para-xylene from a mixture of para-xylene and at least other C8 aromatic, wherein less energy is used to separate desorbent from C8 aromatics.